1. Field of the Invention
This invention relates in general to analog integrated circuits in telecommunication systems, and more particularly to a bias technique for operating point control in multistage analog integrated circuits.
2. Description of Related Art
Analog integrated circuits (IC), such as differential amplifiers, integrated mixers, and buffers, have been widely used in telecommunication systems. One of the desirable features is to operate the parameters of the circuit, such as an average output voltage level and an input stage transconductance, over widely varying process parameters, supply voltages, and temperatures.
In existing multistage analog ICs, bias conditions of all stages are generally set by one current source. This current source controls an input stage transconductance (GM). This current source also controls a quiescent output voltage, such as an output common mode voltage (VOCM) at the output stage of the circuit. Accordingly, any change in the current source for the purpose of affecting an input stage transconductance (GM), for example, increasing GM to improve the performance of the circuit, also affects an average output voltage level, such as an output common mode voltage (VOCM). This is an undesirable feature in many cases, especially since large changes in the current source are usually required to change an input stage transconductance (GM) due to a square root function between GM and I (GM=SQRT(I*Mu*Cox*W/L), where Mu is mobility, Cox is gate capacitance, and W/L is the geometry of a transistor, for example, M1 as described below in FIG. 2), whereas an output common mode voltage (VOCM) is determined by a linear function between VOCM and I (VOCM=VDDxe2x88x92(I*R)/2).
A typical analog integrated circuit (IC) is shown in FIG. 1 which has an input stage, an intermediate stage, and a load stage. An exemplary implementation having a cascoded differential amplifier with resistive loads is shown in FIG. 2. The term xe2x80x9ccascodedxe2x80x9d is different from the term xe2x80x9ccascadedxe2x80x9d. The term xe2x80x9ccascodedxe2x80x9d is generally referred to as the arrangement of several components of a single device being connected to in a series of stages, one on top of another, for example an input stage, an intermediate stage, and an output stage, etc. The term xe2x80x9ccascadedxe2x80x9d is generally referred to as the arrangement of two or more devices being connected in series, one after another.
FIG. 2 illustrates an exemplary differential amplifier having an input stage, an intermediate stage, and an output stage. At the input stage, a differential input pair of transistors M1-M2 and current mirror transistors M3-M4 form an input stage transconductance. The cascodes, transistors M5-M6, form a current buffer at an intermediate stage. Resistors R1-R2 form a load at an output stage.
As shown in FIG. 2, the bias conditions of all three stages are set by one current source I1, including an input stage transconductance GM (GM=SQRT(I1*Mu*Cox*W1/L1) and a quiescent output voltage VOCM (VOCM=VDDxe2x88x92(I1*R1)/2), wherein VDD is a voltage supply, Mu is the mobility, Cox is a gate capacitance, and W1/L1 is a geometry of a transistor M1. Any changes in I1 for the purpose of affecting the input stage transconductance GM also affect the quiescent output voltage VOCM. This is an undesirable feature in many cases, especially since large changes in I1 are required to change GM due to the square root function between GM and I1, thereby causing much larger changes in VOCM due to the linear function between VOCM and I1.
It is with respect to these and other considerations that the present invention has been made.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a bias technique for operating point control in multistage analog circuits.
The present invention solves the above-described problems by providing a technique of independently controlling a bias current in each stage of a multistage analog circuit. This technique allows independent control of parameters, such as an average output voltage level and an input stage transconductance. Accordingly, any changes of a current source at an input stage for the purpose of affecting an input stage transconductance would not affect an average voltage level at an output stage.
In one embodiment of the present invention, a multistage analog circuit for independently controlling a bias current in each stage of the multistage analog circuit having an input stage, an intermediate stage, and an output stage, includes a first current source which controls the input stage of the circuit, a second current source which controls the intermediate stage of the circuit, and a third current source which controls the output stage of the circuit. The bias current in each stage of the circuit is set by the first, second, and third current sources, wherein an output voltage of the circuit is capable of remaining the same when the first current source is changed to affect a transconductance of the input stage.
Still in one embodiment, the bias current in the input stage is determined by the first current source.
Further in one embodiment, the bias current in the intermediate stage is determined by the first and second current sources.
Additionally in one embodiment, the bias current in the output stage is determined by the first, second, and third current sources.
Yet in one embodiment, the multistage analog circuit can be a differential amplifier, an integrated mixer, a buffer, or any other suitable multistage analog circuits.
In one embodiment of the present invention, a method of independently controlling a bias current in each stage of a multistage analog circuit having an input stage, an intermediate stage, and an output stage, includes the steps of providing a first current source which controls the input stage of the circuit, a second current source which controls the intermediate stage of the circuit, and a third current source which controls the output stage of the circuit; changing the first current source to change a transconductance of the input stage; and setting the second and third current sources such that an output voltage of the circuit remains the same.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.